IIRC the sprocket won't go through the head. I don't know if there is a risk of the chain slipping off an unseen sprocket. What I did ages ago is use a third hand to keep tension on the chain while lifting the head then use the third hand to pull the chain under the head when the head was several inches above the block.

How about new exhaust manifold nuts? I believe they use deformed copper nuts that don't hold so well the second time around.

Do you have the offset hex bit that lets you get to the head bolts blocked by the cam?

http://www.shoptoolsshoptools.com/shopexd.asp?id=2912

Sixto
93 300SD